Silver halide photographic elements contain silver halide crystals as a light sensitive substance. Silver halide crystals have a native sensitivity only to blue light. To make the crystals sensitive to other wavelengths of light, they are typically dyed with a spectral sensitizing dye or combinations of such dyes, such as cyanine-type dyes. These dyes capture the light energy and transfer it to the silver halide, forming a latent image which can be amplified when the material is processed. Since the viewed image in the photographic material is provided by colored image dyes in color systems, and by silver in black-and-white systems, it is also important that the senitizing dyes be effectively removed during processing of the element. Retained sensitizing dyes lead to degraded colors and stains in white areas of photographic prints.
In recent years advances in photographic efficiency have been realized through the use of so-called tabular grains. The thin flat geometry of these grains allows more crystals to be coated for the same mass of silver. This advantage can be used to reduce the total amount of silver, for example; or to reduce the graininess of photographs produced from photographic elements that contain these grains. Another advantage of tabular grains is the increased surface area per mass of silver. This allows the adsorption of larger quantities of spectral sensitizing dyes per mass of silver. In this way more light of wavelengths beyond the native sensitivity of the silver halide can be absorbed, further increasing the sensitivity of the silver halide to green, red, and infrared light.
However, the use of tabular grain emulsions in photographic elements is not without difficulty. In particular, it has been found that chemical sensitization (the addition of impurities such as sulfur and gold to the silver halide crystals to enhance their native or intrinsic response to blue light) is more difficult to control than with other grain types, such that higher photographic sensitivity is difficult to achieve. This problem has frequently been overcome by using a technique known as dye-in-the-finish. In this technique, the spectral sensitizing dyes are added in some fashion to the silver halide grains prior to the addition of the chemical sensitizers. In this way the dyes help to control the "finishing" of the grains and an improved response is achieved. Such technique is described in paragraph IIIB of Research Disclosure I (referenced later in this application).
Practically, it is not sufficient to simply achieve a higher sensitivity to light. Once the finished silver halide grains are incorporated into a photographic element, that element must have the proper speed, contrast, and fog for its intended application, and these properties must be stable during storage. Again because of the high surface area of tabular grains exposed to environmental factors such as oxygen and moisture, a stable finish with respect to speed and fog is sometimes difficult to achieve even with the dye-in-the-finish technique. This is particularly important with tabular emulsions which have been sensitized to green light, since most color photographic materials have red, green, and blue sensitive layers and many black-and-white materials are also sensitive to green light. For these reasons, there is still a need for improved dyes and combinations of dyes for spectrally sensitizing tabular emulsions to green light.
Also, recently, there has been a need to provide photographic materials that can be processed in shorter times using more concentrated processing solutions. This move to shorter process times has led to an increased problem with stain in processed photographic materials caused by the retention of sensitizing dyes. This can be of particular concern with tabular emulsions because of the increased amount of sensitizing dye per mass of silver as stated above. Higher levels of sensitizing dye combined with shorter processing times aggravate sensitizing dye stain to the extent that many dyes which efficiently act as spectral sensitizers can not be used practically. Therefore, there is even a greater need for spectral sensitizing dyes or combinations of dyes for tabular silver halide emulsions that provide optimum sensitivity to light, acceptable levels of fog, excellent stability during storage, and low levels of retained dye after processing.
The most commonly used class of spectral sensitizers for the green region of the spectrum is the oxacarbocyanine class of dyes. By themselves these dyes provide good sensitivity to light with wavelengths between 525 nm and 555 nm. Those dyes have also been used in combination with other dyes. For example, naphthoxazole carbocyanine dyes and benzimidazole carbocyanine dyes have been used in combination with oxacarbocyanine dyes. Also, UK 1,231,079, U.S. Pat. Nos. 4,544,628, 4,607,005, and 4,701,405 all describe the use of combinations of oxacarbocyanine dyes and benzimidazolocarbocyanine dyes. However, the foregoing references do not use such combinations on tabular grain emulsions or use combinations which do not provide good light senstivity, low fog levels, good storage stability and low retained dye levels following processing. (Also, EPA 126990 describes a particular type of benzimidazole dyes in combination for improved keeping).
It would therefore be desirable to provide photographic elements having tabular silver halide emulsions, with green sensitizing dyes which provide good sensitivity, low fog, good storage stability, and low levels of retained sensitizing dye, and which may be added to the emulsion prior to chemical sensitization.